What does Dujiangyan "fish mouth" mean?

As one of the three main projects of Dujiangyan water conservancy project, the fish mouth is used to divide the water in Minjiang River into two parts, so that it can divert water, discharge flood and discharge sand. The word "fish mouth" has been used since the Ming Dynasty. It was first seen in Dujiangyan Iron Cow Monument written by Gao Shao in Jiaqing of Ming Dynasty: "Where two cows are made of 67,000 Jin of iron, they stand in the middle of the weir and make the two rivers surge. Three iron piles were erected downstream of the cow to fix the fishmouth stone, and bamboo cages and rolls were still placed under the mouth to protect it as usual, so that the river channel was fixed and the weir bodies were connected. The fish mouth here seems to refer to the front end of the present Jingang dike, and the name of the fish mouth has been used to this day. The modern "fish mouth" refers to the front end of King Kong Dike. For example, Dujiangyan Water Conservancy in guanxian also refers to the front end of Jingang Dike as fish mouth in the project introduction and project location map. The author thinks that the specific meaning of "fish mouth" may be a misunderstanding. The reason why the ancients named the water diversion project after the "fish mouth" is closely related to the real fish mouth. As long as you pay a little attention, it is not difficult to observe that fish breathe through gills in water, and water enters from the mouth and comes out from the gills. This inlet and outlet divide water into two parts. Isn't this in line with the "diversion and diversion" of the fish mouth project? From the principle of fish mouth breathing, isn't this water diversion project from the intersection of the central line of the inner and outer rivers to the rivers on both sides of the tail of Jingang levee just to build a section of Minjiang River into a huge fish mouth? It makes the water of the whole Minjiang River swallow from this fish mouth, and then spit it out through two gills. The left gill flows into an outer river and the right gill flows into Neijiang. The center line of this fish mouth points to the right bank at the bend of Minjiang River, so that the center line of the left gill is aimed at the center of the river. During the flood season, the mainstream flows into the left gill and Jinma River, discharging a lot of floods and sediments. The whole horizontal position of the fish mouth leans to the right. In the dry season, the river can flow into Neijiang as much as possible through the right gill. It plays the role of "falling apart and leveling the wasteland". The head and downstream of the canal pass through the secondary and tertiary fish mouths of different sizes from Neijiang, forming an irrigation network with dense tributaries to irrigate the fertile fields of the land of abundance. The ancients named the water diversion project, which must have been inspired by the fish mouth, but it was misinformed for various reasons. The front end of Jingang levee is well-deserved as the key project of water intake project, but I think it is inappropriate to regard it as a "fish mouth". Treating the front end of King Kong Dike as a fish mouth means treating King Kong Dike as a fish. The Jingangdi is in the river and looks like a fish in appearance. Fish can only swim in the water, but it doesn't have the function of dividing water, because it is very small compared with the water it swims in. I think it is understandable that the front end of Jingang levee is regarded as the "fish mouth" theory of the whole project, because it is not only the key to water diversion, but also the most easily damaged part. But as a project, it should be understood not only from the appearance, but also from the function. In practice, we have long regarded "fish mouth" as one of the three major components of Dujiangyan water diversion project. Jingang levee and its front end are only the key facilities of this project. It is difficult to achieve the water diversion function satisfactorily without the proper cooperation between the complete outer diamond dike and the flow curve. If we only look at the key parts as a whole, it is tantamount to ignoring the white parts when we look at a landscape plan. A correct understanding of fish mouth is of great significance to our understanding of Dujiangyan water diversion project.